For over one hundred years photographic films have been used to capture and display X-rays for diagnostic purposes. In the last ten years or so, digital radiography has become increasingly popular. Digital radiography refers to the application of digital image processing techniques to projection radiography (x-rays). Digitally recorded x-rays are superior to those recorded with photographic film due to the greater dynamic range of the digital recording system. Furthermore, computer image processing techniques provide a wealth of capabilities to study otherwise obscured details within the image.
To take a digital radiograph, a digital radiography imaging unit is positioned behind a subject. A standard radiographic generator directs radiation through the subject to a fluorescent-imaging screen mounted just behind the front surface of the imaging unit. The imaging screen is the conversion media for radiation to visible light. The fluorescent-imaging screen absorbs the radiographic radiation and emits light of a particular wavelength which closely matches the peak sensitivity of a charge coupled device (CCD) camera. A front-surfaced mirror is positioned at a 45 degree angle inside the imaging unit to direct the radiographic image into the CCD) camera. The mirror allows the CCD camera to be positioned out of the direct path of the radiation, effectively shielding it from radiation exposure and prolonging its life. A high-efficiency lens reduces the image and directs it onto the surface of the CCD.
The visual image formed by the fluorescent-imaging screen is converted into a digital image by the CCD sensor. A control computer converts the image into a medical image file that can be viewed for clinical diagnosis, enhanced and electronically stored with the patient demographic information in a picture archiving system.
In the digital x-ray imaging unit described above, it is desired to place the lens as close as possible to the fluorescent screen in order to capture as much light as possible. In so doing, the required dose of radiation to the patient is reduced. However, such positioning of the lens requires the lens to have a very wide angle and be very fast, i.e. have a large aperture. When an image is taken using such a lens, distortions are typically introduced into the image. Also, distortions can appear in other digital imaging applications, especially where the lenses used have a wide angle and/or have a large aperture. While such distortions do not represent a loss of data, but rather the misplacing of it, it is desirable to view a distortion-free image.